The reason we don't do that here is because it would make people stay clear of those threads because it states it has been answered. However, 1) the answer, even though it might work, may be wrong or incomplete (e.g. working PHP code, but vulnerable to SQL Injection, or some other kind of attack), or 2) there might be much better/more elegant answer out there that the person who "solved" the thread didn't know about (e.g. a neat CSS solution instead of jQuery with a gazillion plugins).
So, by not applying a kind of "resolved" state we hope to keep the thread open for better and more complete answers